The present invention relates to a permanent magnet rotating electric machine that has a rotor built with a plurality of permanent magnets embedded in a rotor core.
A conventional permanent magnet rotating electric machine is disclosed, for example, in JP-A-9-327140. In such a permanent magnet rotating electric machine, the rotor core is fixed on the shaft and the rotor is built by inserting a plurality of permanent magnets, each with a rectangular cross section, from the shaft direction into the storage section formed on the rotor core so that the rotor can rotate with a predetermined gap to the inner periphery of the stator core within the stator. The permanent magnets are magnetized such that the north pole and the south pole alternate.
A motor with rectangular magnets embedded in the rotor, such as the one described above, is efficient during high-speed rotation because field weakening is effective during high-speed rotation. For this reason, the motor is used, for example, as the permanent magnet motor on an electric car where high-speed rotation is required. To avoid vibrations and noises generated during driving, the magnets within the rotor are split in the longitudinal direction to produce semi-slot skews for attaining low-torque pulsation.
The motor torque of a magnet-embedded rotating electric machine such as the one described above is expressed by expression (1) given below.T=φIq+(Lq−Ld)Iq×Id  (1)where, T is the motor torque, φ is the magnetic flux of the permanent magnet, Lq is the q-axis inductance, Ld is the d-axis inductance, Iq is the q-axis winding current, and Id is the d-axis winding current.
In expression (1), the first term is the torque of the major magnetic flux of the permanent magnet, and the second term is a reluctance torque generated by the auxiliary magnetic pole of an iron core between two magnets. The magnetic torque has a period for each pole pair (electrical angle of 360 degrees), while the reluctance torque has a period for each pole (electrical angle of 180 degrees).
In this case, if the rotor is skewed as in the prior art example, the maximum torque is decreased because of a difference in the current phase for generating the maximum torque. If the magnet torque is the main torque, the torque decrease is small, for example, about 5% because the magnetic torque has a period of 360 degrees; however, if the reluctance torque is the main torque, the torque decrease is large, for example, about 10% because the reluctance torque has a period of 180 degrees. Thus, for a motor that uses the reluctance torque as the main toque, there has been a need for a shape that reduces torque pulsation with no skewing.